Hard Water pH Level Vs. Soft Water pH Levels (Acidic Vs. Alkaline)

“Is hard water acidic or alkaline?”

This hard water pH levels question comes up time and time again. To settle it once and for all, here is the quick clear answer:

  • Hard water is alkaline (it has pH levels above 7.0).
  • Soft water is acidic (it has pH levels below 7.0).
is hard water alkaline or acidic with ph paper test
Soft water on the left (red pH indicator color, below 7.0 pH) vs. hard water on the right (blue pH indicator paper, above 7.0 pH).

In many cases, we want to know exactly what the pH of hard water is. Namely, if we know the relationship between water hardness and pH levels, we can just:

  • Measure the water pH levels with a home kit (litmus paper test).
  • Based on that pH, check if we have hard water.

Example: Let’s say you use a home pH kit to measure the pH of the water in your tap. The pH kit says that the pH of your water is 9.0. Do you have hard water or not? If the water pH is 9.0, you quite certainly have hard water.

Now, what is pH of hard water is quite difficult to say for certain. With certainty, we can say that hard water is basic (alkaline); that just means it has a pH level over 7.0.

Science-wise, hard water is alkaline because it contains a larger concentration of calcium ions (Ca2+). Calcium hydroxide (Ca(OH)2) is a very strong base; hence water that contains calcium ions will also have basic (alkaline) properties.

The calcium ions in our water pipes are usually created from calcium carbonate (CaCO3). This following calcium carbonate dissolution equation and hard water pH calculation will help us estimate hard water pH value:

CaCO3 + H2O = Ca2+ + CO2 + 2 OH

pH = 14 – log [OH] = 14 – log [CaCO3/2]

Based on the calcium dissolution pH calculation, we can roughly estimate that (you will find a full water hardness pH chart below):

  • Soft water has a pH level below 7.0. Water that contains less than 60 PPM (Parts Per Million) of CaCO3 or below 3.50 GPG (Grains Per Gallon) of CaCO3 is considered soft water. In the chart below, soft water is marked with this color.
  • Moderately hard water has a pH level ranging from 7.0 to 7.5 pH. Moderately hard water contains 61 to 120 PPM or 3.56 to 7.01 GPG. In the chart below, moderately hard water is marked with this color.
  • Hard water has a pH level ranging from 7.5 to 8.5 pH. If you measure pH within this range, you likely have 121 to 180 PPM or 7.06 to 10.51 GPG water hardness which is categorized as hard water. In the chart below, hard water is marked with this color.
  • Very hard water has a pH level of 8.5 or above. We categorize water that contains above 181 PPM or above 10.57 GPG as very hard water, the kind of water hardness that can severely damage your pipes and water-using appliances like washing machines or water heaters. In the chart below, very hard water is marked with this color.
hard water ph levels and effect on tankless water heater
This is a very complex and thus expensive $4,000 11 GPM gas tankless water heater. Very hard water (with a pH above 8.5) can severely reduce its lifespan.

To help you out, we have created a water hardness pH chart that tells you how many PPM does a sample of water with certain pH has. Here is the chart with the color markings for soft water, moderately hard water, hard water, and very hard water:

Water Hardness pH Chart

PPM (Parts Per Million): pH Level:
0 PPM Below 6.5 pH
1 PPM Below 6.5 pH
5 PPM Below 6.5 pH
10 PPM Below 6.5 pH
15 PPM Below 6.5 pH
20 PPM Below 6.5 pH
25 PPM Below 6.5 pH
30 PPM 6.5 pH
35 PPM 6.6 pH
40 PPM 6.7 pH
45 PPM 6.8 pH
50 PPM 6.9 pH
60 PPM 7.0 pH
70 PPM 7.1 pH
80 PPM 7.1 pH
90 PPM 7.2 pH
100 PPM 7.3 pH
110 PPM 7.4 pH
120 PPM 7.5 pH
130 PPM 7.6 pH
140 PPM 7.8 pH
150 PPM 7.9 pH
160 PPM 8.1 pH
170 PPM 8.3 pH
180 PPM 8.5 pH
190 PPM 8.6 pH
200 PPM 8.7 pH
250 PPM 8.9 pH
300 PPM Over 9.0 pH
350 PPM Over 9.0 pH
400 PPM Over 9.0 pH
450 PPM Over 9.0 pH
500 PPM Over 9.0 pH
550 PPM Over 9.0 pH
600 PPM Over 9.0 pH
650 PPM Over 9.0 pH
700 PPM Over 9.0 pH
750 PPM Over 9.0 pH
800 PPM Over 9.0 pH
850 PPM Over 9.0 pH
900 PPM Over 9.0 pH
950 PPM Over 9.0 pH
1000 PPM Over 9.0 pH

As you can see, all 3 categories of hard water (moderately hard, hard, and very hard) are alkaline. They have an above 7.0 pH levels. Soft water is acidic; it has a below 7.0 pH levels.

Now, it is important to understand that while calcium ion concentration this chart and water hardness scale (with PPM, GPG, mmol/L ranges) are based upon are not the only ions in our water.

There is a whole list of ions dissolved in our water that can impact the pH of water as well. These ions can include:

  • Sodium ions (Na+)
  • Iron ions (Fe2+ and Fe3+)
  • Potassium ions (K+)
  • Magnesium ions (Mg2+)
  • Nitrate ions (NO3-)
  • Phosphate ions (PO43-)
  • Chloride ions (Cl)

It is important to understand that measuring the pH of water only serves as an estimate of water hardness. A more accurate method of checking water hardness at home are home water hardness kits. For further reading about water hardness and pH levels relationship, you can check this scientific article by B√ľker et al.

These include a colorimetric titration with EDTA solution (it’s actually quite similar to measuring pH levels with a litmus paper).

We hope this definitively answers whether hard water is alkaline or acidic. On top of that, we can estimate what is the pH of hard water based on the calcium carbonate dissolution equation and pH calculation. If you have any questions about your water pH levels, you can give us some numbers in the comments below, and we will help you out.

Leave a Comment